1. Field of the Invention
The invention concerns the area of turbomachines and in particular of turbojet engines with their fan attached to a drive shaft which is supported by at least a first bearing.
2. Discussion of the Background
Such a turbojet engine includes, from upstream to downstream in the direction of the flow of the gases, a fan, one or more compressor stages, a compression chamber, one or more turbine stages and a gas-exhaust nozzle. The fan includes a rotor fitted with blades on its circumference which, when they are rotated, drive the air into the turbojet engine. The fan rotor is supported by the shaft of the low-pressure rotor of the engine. It is centred on the axis of the turbojet engine by a first bearing which is upstream of a second bearing connected to the fixed structure, in particular the intermediate housing.
In the remainder of the description, to the extent that the fan is attached to the compressor shaft, which is the shaft of the low-pressure rotor in a twin-shaft engine, this shaft is known by the unique term of “compressor shaft”.
The first bearing is supported by a support element, forming an envelope around the compressor shaft, oriented to downstream of the first bearing and secured to a fixed structure of the turbojet engine. The second bearing is supported by a support element which is also secured to a fixed structure of the turbojet engine.
It can happen that a blade may become detached from the fan accidentally. This results in a severe imbalance in the compressor shaft, which leads to loads and vibrations on the bearings, transmitted by their support elements to the fixed structures of the turbojet engine, which can be damaged as a result.
In order to prevent a risk of excessive damage to the turbojet engine, it is possible to over-dimension the structure or, as in patent FR 2,752,024, to propose a system for uncoupling of the first bearing. The support element of the first bearing is fixed to the structure of the turbojet engine by screws of the fuse or rupture type, which include a weakened section that causes them to break in the event of excessive forces. Thus, on the appearance of the imbalance in the compressor shaft, the forces exerted on the first bearing are transmitted to the rupture screws which break, uncoupling the support element of the first bearing from the structure of the turbojet engine. According to other methods of implementation, the support of the second bearing is associated with that of the first bearing in order to accompany it in the event of uncoupling, or includes its own uncoupling system, independent of that of the first bearing. After uncoupling, the forces created by the imbalance are no longer transmitted to the fixed structure of the turbojet engine by the support elements of the bearing or bearings.
However, after the uncoupling of one or both bearings, the fan continues to rotate, and the compressor shaft can no longer rotate on its axis and undergoes large displacements capable of damaging the fixed structure of the turbojet engine. In this case, patent FR 2,752,024 proposes the provision, on the fixed structure of the turbojet engine, of a stiffening band surrounding the support element of the first bearing, to which, in this case, is attached that of the second bearing, and performing the function of movement limiter or back-up bearing.
The continued rotation of the fan can nevertheless lead to stresses in the compressor shaft and the turbine shaft, which are attached to each other, and can give rise to breakage of one or both of these. In any case, we are speaking of rupture of the compressor shaft. In this case, the rotation of the fan leads to the latter, as well as the compressor shaft to which it is attached toward the front. The fan is then ejected out of the turbojet engine, and this is what has to be prevented.
The band proposed in patent FR 2,752,024 can however, in the event of rupture of the compressor shaft, perform a function of axial retention of the fan rotor, with the fixing bracket of the support element of the first bearing to the fixed structure of the turbojet engine then coming up against a radial wall of this band. However, because of the flexing to which the compressor shaft can be subjected in this situation, an angle can exist between the wall of the bracket and the wall of the band about to abut, resulting in either a rather ineffective stopping of the shaft with damage to the elements through friction, or even, if the angle is too great, to passage of the bracket, inclined radially in relation to the axis of the turbojet engine, beyond the band, therefore making it impossible to stop the advance of the compressor shaft and of the fan rotor, which are then ejected or trapped across its retention fairing, thus damaging the whole structure of the turbojet engine.